1. Field of the Invention
This invention relates generally to magnetic tunneling junction (MTJ) MRAMs and more particularly to the use of an oxidation process that leads to a smooth bottom electrode and resulting superior performance properties.
2. Description of the Related Art
The magnetic tunneling junction device (MTJ device) is essentially a variable resistor in which the relative orientation of magnetic fields in an upper and lower magnetized electrode controls the flow of spin-polarized tunneling electrons through a very thin dielectric layer (the tunneling barrier layer) formed between those electrodes. As electrons pass through the lower electrode they are spin polarized by its magnetization direction. The probability of an electron tunneling through the intervening tunneling barrier layer then depends on the magnetization direction of the upper electrode. Because the tunneling probability is spin dependent, the current depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. Most advantageously, one of the two magnetic layers (the pinned layer) in the MTJ has its magnetization fixed in direction, while the other layer (the free layer) has its magnetization free to move in response to an external stimulus. If the magnetization of the free layer is allowed to move continuously, as when it is acted on by a continuously varying external magnetic field, the device acts as a variable resistor and it can be used as a read-head. If the magnetization of the free layer is restricted to only two orientations relative to the fixed layer (parallel and anti-parallel), the first of which produces a low resistance (high tunneling probability) and the second of which produces a high resistance (low tunneling probability), then the device behaves as a switch, and it can be used for data storage and retrieval (a MRAM).
Magnetic tunneling junction devices are now being utilized as information storage elements in magnetic random access memories (MRAMs). Gallagher et al. (U.S. Pat. No. 5,640,343) discloses an array of MTJ elements connected to a diode in series, the whole comprising an MRAM array. The individual MTJ elements are substantially formed as described briefly above. As is further disclosed by Gallagher, when used as an information storage or memory device, a writing current (or currents) orients the magnetization of the free layer so that it is either parallel (low resistance) or anti-parallel (high resistance) to the pinned layer. The low resistance state can be associated with a binary 0 and the high resistance state with a binary 1. At a later time a sensing current passed through the MTJ indicates if it is in a high or low resistance state, which is an indication of whether its magnetizations are, respectively, antiparallel or parallel and whether it is in a 0 or 1 state. Typically, switching the magnetization direction of the free layer from parallel to antiparallel and vice-versa is accomplished by supply currents to orthogonal conductor lines, one which is above the MRAM cell and one which is below it. The line above the cell is referred to as the “digit” or “word” line and is electrically isolated from the cell by intervening dielectric material. The line below the cell, called the “bit” line, is in electrical contact with the top of the cell and is used for both writing (changing free layer magnetization) and reading (detecting high or low resistance). The word line is oriented so that its magnetic field is along the hard axis of the free layer. The bit line provides the component of the switching field parallel to the easy axis of the free layer. The two lines intersect orthogonally with the cell lying between them so that the combined field peaks just above the switching threshold of the cell (field required for a transition from parallel to antiparallel, or vice versa, relative orientations of the free and pinned layer magnetizations). For fast operation of the cell, it must have a high magnetoresistance ratio (DR/R), where DR represents the resistance variation when the free layer magnetization switches direction and R represents the total minimum resistance of the cell when free and pinned layers are magnetized in parallel directions. For stable operation, the cell's junction resistance, RA, where A is cell cross-sectional area, must be well controlled. When the MRAM device is used as the basic element of a memory, it is replicated to form an array of many such devices and integrated with associated CMOS circuitry which accesses particular elements for data storage and retrieval.
When fabricating an MRAM element or an array of such elements, the necessity of creating a high value of DR/R and maintaining a high degree of control over the junction resistance requires the formation of thin, smooth layers of high quality.
In a standard MRAM array structure the MTJ stack (lower electrode/AlOx tunneling barrier/upper electrode) is deposited on top of the bottom conductor (the bit line), which is a tri-layer such as Ta/Cu/Ta or NiCr/Ru/Ta. The lower electrode is a magnetically pinned layer, the upper electrode is a magnetically free layer and the tunneling barrier layer is a layer of oxidized aluminum. Great efforts have been made in trying to control the oxidation of the aluminum when forming the barrier layer. Typically, the Al layer is formed on the bottom electrode by physical vapor deposition (PVD). This form of deposition produces a thin layer of polycrystalline aluminum. Depending on the thickness of the aluminum layer, different forms of oxidation have been used. For example, natural oxidation (NOX), plasma oxidation and radical oxygen oxidation (ROX) have all been used to produce the barrier layer and will be discussed further below.
A typical tunneling barrier layer for an MRAM device is made by in-situ oxidation of a 7 to 10 angstrom thick Al layer and the resulting oxidized layer has a junction resistance, RA, in the kilo-ohm-micron2 (kΩμm2) range. When the oxidation method used is plasma oxidation, the energetic oxygen plasma ions may damage the underlying ferromagnetic material of the lower (pinned) electrode. Therefore, radical oxidation (ROX) is normally used for the oxidation method. ROX is achieved by covering the plasma with a grounded metal mesh “shower cap,” so that only the oxygen radical and neutral oxygen can reach the substrate. In the initial stage of ROX, oxygen covers the aluminum grain surface homogeneously and the resulting oxidized structure is amorphous. Oxidation starts at the Al surface and forms a good Al2O3 stoichiometry. The oxidation process then moves progressively downward to the interface between the deposited aluminum layer and the underlying electrode surface. It is known that oxygen diffusion proceeds much more rapidly along the grain boundary than into the grain itself. Consequently, the oxygen diffusion front does not proceed at a uniform rate or with a uniform spatial dependence through the deposited aluminum layer. A discussion of several oxidation methods can be found in Y. Ando et al., “Growth mechanisms of thin insulating layer in ferromagnetic tunnel junctions prepared using various oxidation methods,” J. Phys. D.: Appl. Phys., Vol. 35, 2415–2421, (2002).
When the Al layer is “under-oxidized,” meaning that the portion of the layer closest to the interface with the bottom electrode has an oxide stoichiometry of the form Al2Ox, with x<3, as is the case in NOX (without the assist of a plasma), the product RA is low, and so are the layer breakdown voltage and the GMR ratio, DR/R. When the layer is “over-oxidized,” meaning that oxygen has diffused into the lower electrode, the product RA is greatly increased, but DR/R is decreased. The use of plasma oxidation to produce oxidized layers is discussed in Heejae Shim et al., “Magnetic tunnel junctions with a tunnel barrier formed by N2O plasma,” Appl. Phys. Lett., Vol. 83, No. 22, p. 4583, 1 Dec. 2003.
After the MRAM film stack is formed (the multi-layered lamination of layers before patterning), it is thermally annealed to fix the magnetiization direction of the pinned ferromagnetic layer (the lower electrode). Annealing improves the homogeneity of the oxidized aluminum layer by redistributing the oxygen in the barrier layer and by driving out the oxygen from the pinned layer. It is found that the thermal annealing process improves the integrity of the barrier layer (eg. it raises the breakdown voltage) and enhances the DR/R ratio.
In order to obtain a high DR/R, the ferromagnetic layers adjacent to the barrier layer are formed of CoFe. It has been reported that CoFe with 25% Fe by number of atoms yields a higher DR/R than CoFe with 10% Fe by number of atoms. This is due to the fact that the CoFe(25%) produces a higher degree of spin polarization of conduction electrons at the CoFe/Al2Ox interface. Experiments have also shown that an electrode formed of NiFe(60%) also gives a high DR/R and, in addition, because the binding energy between Fe and O is weaker than that of Co and O, the annealing process drives oxygen out of a NiFe electrode more readily than a CoFe electrode.
In the fabrication of MTJ MRAM devices it would be highly desirable to form an Al2Ox tunneling barrier layer that is flat and smooth and has an Al2O3 stoichiometry at its upper and lower interfaces with the electrodes. In this case, the spin polarization would be symmetrical at both interfaces, yielding both a high DR/R and breakdown voltage.
The present invention discloses a novel oxidation technique for forming an improved Al2Ox barrier layer. The invention is to do the oxidation from both sides of the deposited Al layer, so that the Al2O3 stoichiometry is ultimately more uniformly achieved within the entire body of the layer. The method that is proposed to produce this symmetrical oxidation process is to form an oxygen surfactant layer (OSL) on the surface of the CoFe(25%) or NiFe(60%) pinned layer, so that there is a source of oxygen for the bottom surface of the deposited Al layer. An oxygen surfactant layer, which is discussed in related Applications HT 02-032and HT 03-006, both of which are fully incorporated herein by reference, is a sub-monolayer of oxygen adsorbed on the surface of a deposited layer. When the Al layer is then subjected to a ROX process, the plasma supplies oxygen to the exposed Al surface, while the surfactant layer supplies it to the “hidden” surface. It is noted that reactivity of oxygen is stronger with Al than with CoFe, so the oxygen in the OSL will diffuse into the Al layer to form a CoFe/Al2O3 interface.